feat(aptu): add ai block to aptu.yml (required after aptu-github-app#61)

[clouatre]

Summary

After clouatre-labs/aptu-github-app#61
merges, the Worker no longer falls back to operator-provided AI credentials. All repositories must
supply their own AI configuration via an ai block in .github/aptu.yml. Repositories without
this block will silently receive no triage or review dispatch.

This PR adds the required ai block before that breakage lands.

Change

Adds to .github/aptu.yml:

ai:
  provider: gemini
  model: models/gemini-3.1-flash-lite
  api-key-secret: GEMINI_API_KEY

GEMINI_API_KEY is an org-level Actions secret already available to all repositories in
clouatre-labs. No repository-level secret setup is required.

Relates to

• clouatre-labs/aptu-github-app#61 (removes operator key fallback -- makes ai block required)
• clouatre-labs/aptu-github-app#59 (removes ALLOWED_OWNERS gate)

[aptu-dev]

Aptu Review

✅ Approve — This PR updates the .github/aptu.yml configuration to include the mandatory 'ai' block. This change ensures continued functionality for triage and review services following the removal of operator-provided credential fallbacks in the upstream aptu-github-app.

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